For various reasons, the corneal portions of eyes must be surgically repaired or replaced. For example, the cornea may become scratched or scarred or otherwise physically damaged, greatly hindering sight. The cornea is also subject to the effects of various degenerative diseases, mandating replacement if the patient is to have normal or even near normal vision
The cornea of the human eye is a specialized structure made up of substantially parallel, relatively compacted layers of tissue. The outermost or most superficial layer of the cornea is the epithelial layer. This is a protective layer of tissue which regenerates if injured. Moving inwardly in the eye is the base surface of the epithelial layer known as Bowman's membrane. Immediately adjacent the Bowman's membrane is the stroma of the cornea, which is an extra-cellular collagen architectural matrix with scattered keratocytic cells. The stroma layer is bounded at its deepest level by a cuticular, acellular membrane, referred to as Descemet's membrane, which is followed by a monolayer of single cell thickness of specialized endothelial cells which forms the posterior surface of the cornea. The endothelial layer does not regenerate and when it is diseased, scratched or otherwise injured, it must be replaced.
When disease or injury affect only the mid- or superficial stromal tissue and epithelial layer, as in certain scars, replacement of the superficial or a partial thickness of the anterior stroma may be sufficient to rehabilitate vision in the eye and a lamellar keroplasty or partial thickness transplant can be used. When, however, deep stroma and especially when the endothelium is diseased, those layers must be replaced and a full thickness or penetrating transplant is necessary.
A partial thickness transplant typically involves the replacement of a variable thickness of the outermost layers of the cornea but does not include replacement of the deep lining of Descemet's membrane and endothelial cell layer. The diseased tissue is replaced with an implant which may be a graft of tissue taken from the healthy cornea of a donor eye similar in size and shape to the tissue removed from the patient's cornea ("lamellar graft") or an artificial implant similar in size and shape to the tissue removed from the patient's cornea and made of a biologically acceptable material.
A full thickness transplant typically involves the replacement of all layers of the cornea, including the deep Descemet's membrane and the layer of endothelial cells. A cylindrical plug of the corneal tissue is replaced with a cylindrical implant which may be a plug of tissue cut from a donor cornea through the full thickness of corneal tissue ("penetrating graft") or an artificial implant sized and shaped to fit in a bore surgically formed through a patient's cornea.
The shape of the outermost surface of the cornea influences the quality of vision and changes made in the curvature of that surface can result in regular or irregular astigmatism or a change in the refractive state of the eye. Therefore, it would be desirable to be able to replace the deepest corneal layers without having to replace healthy tissue on the outermost surface of the cornea and to minimize deformation of the outermost corneal surface when all layers of corneal tissue must be replaced.
Furthermore, in India and many other areas of the world there is a great need for lamellar donor tissue due to a variety of factors, such as a higher incidence of superficial eye injuries and trauma to the stroma due to infections and the like; there generally is not as great a need for endothelial transplantation in these areas. Conversely, in the U.S. and other industrialized nations, the eye care tends to be better and the need for endothelial tissue can be significantly greater than the need for lamellar transplants. Furthermore, while supply of donor corneas is generally sufficient to meet demand in industrialized nations, in other countries the demand far outstrips supply. It would obviously be helpful if countries with sufficient supply could deliver tissue to other, more needy areas. Unfortunately, though, corneal tissue generally cannot be preserved for extended periods of time because it cannot be frozen--although the stroma in the epithelial layer can generally be frozen, freezing tends to damage the endothelial cells.